1. Field of the Invention
The current invention describes the use of framework-substituted zeolitic catalysts that are synthesized by substituting framework heteroatoms for B on the external surface of a zeolitic material. In a particular embodiment of this invention, the heteroatom is Ti and the Lewis-acid catalyst is a solid epoxidation catalyst that uses organic hydroperoxide as oxidant. Also described are the surprisingly advantageous properties of the resulting epoxidation catalyst in terms of its accessibility, activity, selectivity, and general robustness, all of which crucially depend on the nature of the zeolitic framework.
2. Description of the Related Art
Zeolites demonstrate extraordinary catalytic utility due to their well-defined catalytic active sites consisting of heteroatoms substituted within the zeolitic framework as well as shape selectivities. However, zeolites have been limited to microporous frameworks in the past, which has limited reactant substrates to small molecules. Incorporating greater accessibility into zeolite catalysts would be invaluable to expanding the scope of their catalysis to include larger and sterically more bulky substrate and product molecules.
MWW layered zeolite precursors, when substituted with metal heteroatoms, have shown catalytic activity using sterically bulky reactants, such as Ti-catalyzed epoxidation of cyclooctene using tertbutylhydroperoxide as oxidant; Al-catalyzed cracking of 1,3,5-triisopropylbenzene, and Sn-catalyzed Baeyer-Villiger oxidation of 2-adamantanone (Wang, L.; Wang, Y.; Liu, Y.; Chen, L.; Cheng, S.; Gao, G.; He, M.; Wu, P. Microporous and Mesoporous Materials 2008, 113, 435; Wang, Y.; Liu, Y.; Wang, L.; Wu, H.; Li, X.; He, M.; Wu, P. Journal of Physical Chemistry C 2009, 113, 18753; and Liu, G.; Jiang, J.-G.; Yang, B.; Fang, X.; Xu, H.; Peng, H.; Xu, L.; Liu, Y.; Wu, P. Microporous and Mesoporous Materials 2013, 165, 210.) Another promising approach for synthesis of accessible zeolites is the transformation of three-dimensional UTL germanosilicate into a two-dimensional lamellar zeolite by Cejka et al., who demonstrated that layers are separated during hydrolysis of the double-four ring (D4R) bridging units by hydrolysis (Roth, W. J.; Shvets, 0. V.; Shamzhy, M.; Chlubna, P.; Kubu, M.; Nachtigall, P.; Cejka, J. Journal of the American Chemical Society 2011, 133, 6130; and Chlubna, P.; Roth, W. J.; Greer, H. F.; Zhou, W.; Shvets, 0.; Zukal, A.; Cejka, J.; Morris, R. E. Chemistry of Materials 2013, 25, 542.) This latter approach, while elegant, requires precursors to consist of D4R units in the space between layers, such that D4R removal via hydrolysis results in two-dimensional zeolite layers, and has only been synthetically demonstrated on zeolite UTL.
Borosilicate zeolites have historically been generally considered to be less useful for acid-catalyzed reactions because their intrinsically weak acidity can effectively catalyze reactions that require mild acidity (Millini, R.; Perego, G.; Bellussi, G. Topics in Catalysis 1999, 9, 13; Chen, C. Y., Zones, S. I., Hwang, S. J., Bull, L. M. In Recent Advances in the Science and Technology of Zeolites and Related Materials, Pts a-C; VanSteen, E., Claeys, M., Callanan, L. H., Eds. 2004; Vol. 154, p 1547; and Chen, C. Y., Zones, S. I. In 13th International Zeolite Conference; Galarneau, A., Di Renzo, F., Fujula, F., Vedrine, J., Eds.; Elsevier: Amsterdam, 2001, p paper 26.) However, borosilicate zeolites provide a unique route for synthesizing many types of isomorphous forms of zeolites at certain Si/M ratios (M=Al, Ga, Ti, etc.), which offer opportunities for synthesizing heteroatom-substituted metallosilicate zeolites, where the metal ions might otherwise be difficult to incorporate into the framework during direct synthesis (Chen, C. Y.; Zones, S. I. In 13th International Zeolite Conference; Galarneau, A., Di Renzo, F., Fujula, F., Vedrine, J., Eds.; Elsevier: Amsterdam, 2001, p paper 11.) In such a modification of one framework metal for another, the B atoms template certain T-positions in the zeolitic framework, and silanol nests can be created upon deboronation (Deruiter, R.; Kentgens, A. P. M.; Grootendorst, J.; Jansen, J. C.; Vanbekkum, H. Zeolites 1993, 13, 128; and Hwang, S. J.; Chen, C. Y.; Zones, S. I. Journal of Physical Chemistry B 2004, 108, 18535.)
Aluminum (Al) heteroatoms have been exchanged or substituted for boron (B) heteroatom in zeolites for many years. This exchange changes a weak acid zeolite into one that is more highly acid. Catalysis by acid sites can impact rates of chemical reaction, rates of mass transfer, selectivity to products and deactivation of the catalytic site or pore system. Better control of the acid sites would help to provide selective control of the overall catalysis.
Though substitution of aluminum for boron has previously been used, the result has been the extremes: the use of 10-MR zeolites where essentially no heteroatom exchange occurs (e.g., ZSM-11) or the use of large- or extra-large pore zeolites where essentially all B heteroatoms are exchanged (e.g., SSZ-33). See, for example, Chen, C. Y.; Zones, S. I., “Method for Heteroatom Lattice Substitution in Large and Extra-Large Pore Borosilicate Zeolites,” U.S. Pat. No. 6,468,501 B1, Oct. 22, 2002; Chen, C. Y.; Zones, S. I., “Method to Improve Heteroatom Lattice Substitution in Large and Extra-Large Pore Borosilicate Zeolites,” U.S. Pat. No. 6,468,501 B1, Sep. 14, 2004; Chen, C. Y.; Zones, S. I. In Studies in Surface Science and Catalysis”; Galarneau, A., Fajula, F., Di Renzo, F., Vedrine, J., Eds.; Elsevier: 2001; Vol. 135; Chen, C. Y.; Zones, S. I. In Zeolites and Catalysis; and {hacek over (C)}ejka, J., Corma, A., Zones, S. I., Eds. 2010, Vol. 1, p. 155. In these instances, acidic conditions are preferred to prevent dissolution of Si from the framework. In the aqueous Al(NO3)3 solution used, the hydrated aluminum cations used in the Al-exchange are too large to enter the 10-MR pores such as ZSM-11. See, for example, Chen, C. Y.; Zones, S. I. In Studies in Surface Science and Catalysis, Galarneau, A., Fajula, F., Di Renzo, F., Vedrine, J., Eds., Elsevier: 2001, Vol. 135; Chen, C. Y.; Zones, S. I. In Zeolites and Catalysis, {hacek over (C)}ejka, J., Corma, A., Zones, S. I., Eds. 2010, Vol. 1, p. 155. In the Al-exchange of B-SSZ-33, the Si/B values increase from 18 to more than 200, and Si/A1 values from 12 to 24, indicating exchange of most B heteroatoms for Al. See, Chen, C. Y.; Zones, S. I. In Studies in Surface Science and Catalysis, Galarneau, A., Fajula, F., Di Renzo, F., Vedrine, J., Eds., Elsevier: 2001, Vol. 135. The result is that either all or none of the boron was exchanged. No selective control is possible.
Catalysis by MCM-22, an aluminosilicate containing Al heteroatoms throughout the lattice framework, and therefore in all three pore systems, is characterized as between a large- and a medium-pore zeolite because it consists of both 10-MR (medium) and 12-MR (large) pores. The role of the acid sites on the external surface hemicages has been determined to differ from those of the internal pore systems through experiments that poison or coke (i.e., formation of carbonaceous deposits in the pore system) the catalytic sites. See, Laforge, S.; Martin, D.; Paillaud, J. L.; Guisnet, M. J. Catal. 2003, 220, 92; Laforge, S.; Martin, D.; Guisnet, M. Microporous Mesoporous Mater. 2004, 67, 235; Laforge, S.; Martin, D.; Guisnet, M. Appl. Catal. A: Gen. 2004, 268, 33; Matias, P.; Lopes, J. M.; Laforge, S.; Magnoux, P.; Guisnet, M.; Ram{hacek over (o)}a Ribeiro, F. Appl. Catal. A: Gen. 2008, 351, 174; Matias, P.; Lopes, J. M.; Laforge, S.; Magnoux, P.; Russo, P. A.; Ribeiro Carrott, M. M. L.; Guisnet, M.; Ram{hacek over (o)}a Ribeiro, F. J. Catal. 2008, 259, 190.
To selectively be able to use acid sites on the external surface would greatly improve one's ability to control a catalysis, and would be of great value to the industry. Moreover, to be able to further enhance a particular reaction by selecting the correct framework of the catalyst would yield even greater benefits to the industry.